In an effort to improve air quality, battery-powered "electric" vehicles are becoming increasingly common in today's automotive marketplace. These electric vehicles typically include traction batteries for supplying electric current to one or more traction motors which, in turn, provide motive power to the vehicle. Accordingly, the vehicle's ability to perform a manoeuvre at any given time at the direction of the vehicle operator directly depends upon the condition of the traction batteries at that time. The condition of each battery, in turn, varies upon such transitory factors as its temperature and its State of charge, as well as other long-term factors as battery aging, polarization effects, etc. Variation in these parameters will substantively affect the manner in which each battery can supply current to the vehicle's traction motors and, hence, the manner in which the vehicle will respond to commands from its operator. Stated another way, in certain situations or under certain load conditions, the vehicle's traction batteries may be unable to meet the transitory current demands of the vehicle which, in turn, will limit the vehicle's temporal capability to perform.
Significantly, the vehicle operator may not realize the qualitative and quantitative limitation on vehicle performance imposed by the condition of the traction batteries. Indeed, these limitations will likely conflict with the operator's own expectations of vehicle performance, given that he will likely have become accustomed to the manner in which vehicles powered by internal combustion engines operate. Specifically, the output of an internal combustion engine and, hence, the responsiveness of a vehicle powered by such an engine, is not noticeably dependent upon the temporal quality of the hydrocarbon-based fuel stored within the vehicle's fuel tank, e.g., the temperature or age of the fuel. Rather, the engine's output is singularly dependent upon the presence of such fuel within the fuel tank. So long as fuel is present, the engine will provide full power upon demand. Indeed, absent an nearly-empty fuel tank, the operator of a vehicle powered by an internal combustion engine will not typically check the vehicle's fuel gauge immediately prior to executing, say, a passing manoeuvre, since it will have no bearing on the vehicle's performance capabilities.
Given the likely expectations of the vehicle operator, there may come a time when he will attempt a manoeuvre that cannot be performed given the amount of current immediately available from the traction batteries, thereby placing the operator, his vehicle and others at great risk of injury. What is needed, then, is a performance monitor for an electric vehicle which can communicate to the vehicle operator, preferably in a familiar fashion, an indication of his present use of available battery "power" so that the operator may then form a reasonable expectation as to the vehicle's further capability to perform at that instant.
Another limitation on the temporal performance capabilities of the typical electric vehicle derives from the correlative use of the traction batteries for supplying current to noncritical electrical components within the vehicle, such as the climate control system's blower fan motor, AC compressor or resistance heater, As a result, the traction batteries are forced to perform "double duty," even in the face of a demand for increased motive performance from the vehicle operator. The increased load from these noncritical components may dangerously limit the amount of current available as for an emergency manoeuvre, again, placing the operator, his vehicle and others at risk. What is also needed, then, is a vehicle performance monitor for an electric vehicle that can shed these noncritical loads on the traction batteries should their current requirements reduce reserve battery current below an acceptable minimum level, given the transitory amount of current then being supplied to the traction motors.